1. Field of the Invention
The present invention relates to a decolorizable toner that can be decolorized by light and a production process for such a decolorizable toner. More particularly, the present invention relates to a decolorizable toner and a production process for such a decolorizable toner that is able to visualize electrical latent images and electrical signals in electronic photographs, electrostatic recording materials and so forth.
2. Description of the Related Art
Recycling and regeneration of used paper has recently been reconsidered for the purpose of protecting the environment, and particularly protecting forest resources, as well as reducing the amount of refuse produced in urban areas. As a part of these reconsiderations, studies are also being conducted on the recycling of waste paper, such as used copy paper, printed matter and facsimile paper, that is produced in corporate offices. With this in mind, corporations have incorporated paper companies within their corporate groups to reprocess and recycle this waste paper by dissolution and production of recycled paper following its collection. However, it is extremely difficult to collect and recycle this paper for a paper company located outside the corporation. Moreover, since the printed portion of printed matter, copy paper and so forth cannot be easily erased, these corporations are forced to discard and dispose of this paper by burning or shredding. Recycling and so forth of this type of paper is therefore considered to be essentially impossible. In addition, since the strength of recycled paper that has been produced using waste paper shredded by a shredder and so forth is generally low, it has the disadvantage of being unable to withstand use as, for example, data forms. Thus, the ideal method of recycling waste paper is one which enables paper to be reused in the office. In order to accomplish this, it is necessary that the printed contents of waste paper be easily erasable.
On the other hand, technically speaking, the development of technologies enabling repeated recording, such as photochromic and thermochromic technologies, has been conducted actively (e.g., Japanese Unexamined Patent Publication No. 60-155179, Japanese Unexamined Patent Publication No. 50-75991 and Japanese Unexamined Patent Publication No. 50-105555). Japanese Unexamined Patent Publication No. 50-75991 in particular discloses a thermally discolorable material that uses a color former consisting mainly of a leuco dye, and a developer consisting of a phenolic hydroxyl group-containing compound. However, although these recording materials are reversibly colored, decolored or discolored by heat and visible light or ultraviolet light, even if the printed portion is decolored, since there is the possibility of it being recolored, they are not suited for irreversibly decoloring the printed portion and reprinting on that same paper.
Therefore, as a result of earnest research in consideration of the above-mentioned related art, the inventors of the present invention disclosed a near infrared light decolorizable recording material and a toner that uses this recording material in Japanese Unexamined Patent Publication No. 4-362935. In the case of performing electrostatic copying using the above-mentioned toner, images, printed characters and so forth that have been recorded onto copy paper can be erased by irradiation with near infrared light. In addition, electrostatic copying can be performed again following erasure to enable this copy paper to be reused, thereby allowing copy paper to be collected and recycled in an office.
However, in the case of the above-mentioned toner, since the dye used demonstrates maximum absorbance in the near infrared light region, absorbance in the readily visible section of the visible spectrum is small, thus having the disadvantage of having low color density. However, if a recording material is used that demonstrates large absorbance in the visible light region to increase color density, its stability with respect to light such as fluorescent light decreases, thus resulting in the practical problem of color fading and printed images being too light.
On the other hand, methods used to fix toner images include a method consisting of fusion and solidification by melting the toner with a heater or heated roller, a method consisting of softening or dissolving the binder resin of the toner with an organic solvent and then fixing onto a support, and a method consisting of fixing the toner onto a support by pressurization. The toner used in the heated roller fixation method is typically prepared by fusing and mixing a colorant such as carbon black and an additive such as an electric charge regulator into a thermoplastic resin such as styrene-butyl acrylate copolymer, so as to be uniformly dispersed, and pulverizing to a desired particle size by a pulverizing machine or dispersing machine after cooling.
In the production process of the decolorizable toner according to this fusion mixing method, cationic dye demonstrating absorbance in the visible and near infrared regions, and additives such as decolorant, heat-resistant aging inhibitor and electric charge regulator, are mixed by high-speed stirring with the binder resin. The resulting mixture is fusedly mixed using means such as a biaxial extruder, heated kneader or heated roller. After cooling, the resulting mixture is pulverized and dispersed as necessary to be able to obtain a toner.
However, in the manufacturing process of the above-mentioned decolorizable toner, the cationic dye breaks down due to heating during mixing of the toner raw materials. This causes the toner to become discolored or faded. In addition, the cationic dye also breaks down when exposed to natural light during storage of the resulting toner, thus also causing the disadvantage of discoloration of the toner.
In addition, another method involves the production of a toner master batch that uses a cationic dye that demonstrates absorbance in the near infrared region, whereby a decolorizable toner is obtained from this master batch. This method is composed of heating, fusing and mixing a binder resin, near infrared absorbing cationic dye-boron anion complex, and as necessary, an anti-discoloration agent, using a biaxial extruder or kneader to be used as the master batch for a decolorizable toner, or the master batch for a decolorizable toner is prepared by cooling the resulting mixture followed by pulverization. Moreover, the mixture resulting from heating and mixing can also be used in following processes as the master batch for a decolorizable toner without cooling, namely in the fused state (Japanese Patent Application No. 5-118633).
However, in production processes using a master batch for the toner, since the binder resin and additives must be further fused and mixed once the master batch has been produced, the number of man-hours increases. In addition, as a result of repeated heating and fusing, the cationic dye in the toner that absorbs from the visible range to the near infrared range tends to break down, thus tending to reduce the uniformity of each component.